The square brackets are the (non-printable) boundaries of the bookmark.
The elimination of the bookmark when you insert text in the range comes from
Word. It does also happen in VBA.

You can do something like this (in pseudo code) in VBA, it should not be
hard to change it to .Net

dim r as range
set r = activedocument.bookmarks(bm_name).range
r.range.text = "hello"
activedocument.bookmarks.add bm_name, r

now the text is in the bookmark (with the brackets). If you want to keep the
bookmark at the beginning you can use something as

r.range.text = "hello"
r.collapse wdCollapseDirectionStart
activedocument.bookmarks.add bm_name, r

success